Recently, negative films of higher sensitivity and broader exposure aptitude have been required with the variation of color negative films into small format styles under various photographic conditions.
Under such circumstances, various proposals have been made for the purpose of improving the photographic properties by the addition of iridium to the silver halide emulsions thereof, including one typical embodiment where silver halide grains are formed in the presence of 10.sup.-8 to 10.sup.-5 mol of iridium per 1 mol of silver, which is to be added to the emulsion at a certain stage of the formation thereof, resulting in various effects of intensification of sensitization, improvement of high intensity reciprocity low failure and improvement of fogging inhibition. These techniques are described in, for example, Japanese Patent Publication Nos. 4935/68 and 32738/70 or Japanese Patent Application (OPI) Nos. 221839/83 and 152438/84 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). In these publications, however, it is reported that the amount of the iridium to be added is preferably 10.sup.-7 to 10.sup.-6 mol/mol AgX and that any sufficient effect cannot be attained when 10.sup.-8 mol or less iridium is added, and further it is additionally reported in the publications that the addition of 10.sup.-5 mol or more iridium results in a noticeable reduction of the sensitivity with no improvement of the characteristics of the emulsions and, therefore, is no more practicable. However, this conclusion almost is based upon the results of examples where at most 10.sup.-4 mol or less iridium is used.
In such background, examples for the preparation of silver halide fine grains containing iridium of high concentration, as found in the present invention, are almost merely those for the measurement of the physical properties of the grains (for example, refer to Journal of Japan Photographic Society, Vol. 31, No. 3, page 139 (1968)), and there are few examples for the evaluation of photographic characteristics of the photographic materials. In fact, there is only one example, where 3.times.10.sup.-4 mol/mol AgX of iridium is added to silver halide grains which are not chemically sensitized. (Refer to Photographic Science and Engineering, Vol. 19, No. 4, page 214 (1975)).
On the other hand, some examples are noted where a large amount of iridium is added to direct positive type silver halide emulsions, as so defined in the present invention. (For instance, refer to Japanese Patent Publication Nos. 37530/75 and 186739/83.) However, the direct positive type silver halide emulsions are those for the formation of positive images by the use of such mechanisms as bleaching the fogged nuclei in the previously fogged surface, and, therefore, are fundamentally different from the other negative type emulsions for the formation of latent images on the surfaces of grains when exposed to light.